The invention relates generally to the field of dynamoelectric machines, and more particularly, a method for securing a permanent magnet to a rotating shaft.
In many dynamoelectric machines, such as self-excited synchronous generators used in aircraft, a permanent magnet generator is utilized as a source of excitation current and includes a circular permanent magnet secured to the machine's shaft. However, a number of problems have been encountered in securing a permanent magnet to a rotating shaft, due primarily to the nature of the materials used for the permanent magnet. For example, the materials used for the permanent magnets, such as Alnico VI, tend to be brittle and in many cases have various metallurgical defects including excessive porosity. One approach to this problem has been to attach the magnet to the shaft by means of circumferential clamping. However, clamping the magnet to the shaft by this method tends to place excessive stress on the magnet, resulting in fractures during the operation of the machine. Another approach has been to use an adhesive material to secure the magnet to the shaft. One disadvantage of this approach results from the temperature-sensitive nature of the adhesive, wherein the adhesive properties tend to fail at elevated temperatures, thus allowing the magnet to become disengaged from the shaft. In many applications this is a serious consideration, since generators, and aircraft generators in particular, quite often are required to function in high temperature environments. Perhaps one of the greatest problems with the adhesive method of attachment results from the differing thermal expansion rate of the steel shaft material and the permanent magnet material. For example, if the magnetic material has a coefficient of expansion greater than the material of the shaft, at high temperatures the magnetic material would tend to expand away from the shaft, thereby placing excessive strains on the adhesive material. On the other hand, if the magnetic material has a coefficient of expansion less than the shaft material, the shaft would expand at a greater rate than the magnetic material at higher temperatures, thus placing unacceptable tensile stress on the magnet, thereby significantly increasing the probability of fracturing the magnetic material.